1. Field of the Invention
The present invention relates to a harmonic high frequency oscillator for generating frequency outputs, which are even-order harmonics of a fundamental wave of an oscillation frequency through so-called push-push oscillation for use in a millimeter-wave band and a microwave band, and more particularly, to a high frequency oscillator which achieves improved phase noise characteristics and frequency pull-in through injection locking.
2. Description of the Related Art
A push-push oscillation based oscillator is known as suitable for generating oscillation signals in a millimeter-wave band and a microwave band. The oscillator based on push-push oscillation based employs a pair of oscillation circuits which operate at the same fundamental frequency but in opposite phases to each other, and combines the outputs from these oscillation circuits to cancel out the fundamental wave component and extract even-order harmonic components to the outside. Such push-push oscillators are used in a variety of applications because of its simple configuration and its ability to generate output frequencies twice or more as high as fundamental wave f0, and are useful, for example, as an oscillation source for a high frequency network which operates, for example, in association with fiber-optic cables, or as an oscillation source for measuring devices. The present inventors have proposed, for example, a high frequency oscillator in Japanese Patent Laid-open Publication No. 2004-96693 (JP, P2004-96693A), which is further reduced in size to facilitate its design and generates, for example, even-order harmonics of second harmonic 2f0 or higher harmonics from fundamental wave f0.
FIG. 1A is a plan view illustrating the configuration of a conventional second-harmonic oscillator for generating a frequency component twice as high as a fundamental wave, i.e., a second harmonic component, and FIG. 1B is a cross-sectional view taken along a line A—A in FIG. 1A.
Basically, a second-harmonic oscillator comprises a pair of amplifiers 3a, 3b for oscillation; microstrip line 1 which serves as a high frequency transmission line within oscillation systems; and slot line 2 for coupling. Slot line 2 functions as an electromagnetic coupler for causing the two oscillation systems to oscillate in opposite phases to each other.
Microstrip line 1 for oscillation is routed on one principal surface of dielectric substrate 5, and ground conductor 6 is formed substantially over the entirety of the other principal surface of dielectric substrate 5. Here, microstrip line 1 is formed in a closed loop substantially in a rectangular shape.
The pair of amplifiers 3a, 3b for oscillation, each comprised of an FET (Field Effect Transistor) or the like, have their output terminals disposed on the one principal surface of dielectric substrate 5 in a mutually opposing relationship, and are inserted in microstrip line 1. In this way, microstrip line 1 connects input terminals of the pair of amplifiers 3a, 3b for oscillation to each other, and the output terminals of the same to each other.
Slot line 2 is implemented by an aperture line formed in ground conductor 6 on the other principal surface of substrate 5, and is routed to vertically traverse two sections in central portions of microstrip line 1 which is routed on the one principal surface of substrate 5. Slot line 2 extends upward and downward by λ/4 respectively from the sections of microstrip line 1 which are traversed by slot line 2, where λ represents the wavelength corresponding to an oscillation frequency (i.e., fundamental wave f0), later described. Microstrip line 4 for output is routed on the one principal surface of substrate 5 and superimposed on slot line 2. Microstrip line 4 is connected to the center of a portion of microstrip line 1 (the lower side in the figure) which connects between the outputs of the pair of amplifiers 3a, 3b for oscillation. Injection line 7 is also connected to a midpoint of microstrip line 1 which connects between inputs of the pair of amplifiers 3a, 3b for oscillation. Injection line 7, which has a microstrip line structure, is arranged to overlie slot line 2.
In the foregoing oscillator, microstrip line 1 is electromagnetically coupled to slot line 2 to form two oscillation systems, as shown in the left and right halves of the figures. In this configuration, a high frequency signal in an unbalanced propagation mode, which propagates through microstrip line 1, is converted into a balanced propagation mode of slot line 2. Since the balanced propagation mode of slot line 2 involves a propagation which presents opposite phases at both sides of the aperture line, eventually causing the two oscillation systems to oscillate in opposite phases to each other. Since the oscillation frequency (i.e., fundamental wave f0) in the oscillation systems generally depends on the length of each oscillation closed loop or on a phase shift amount in the loop, the oscillation systems are designed such that the respective oscillation systems oscillate at the same oscillation frequency.
At the midpoint of microstrip line 1 which connects between the outputs of the pair of amplifiers 3a, 3b for oscillation to each other, the fundamental wave (f0) component and odd-order harmonic components in the oscillation frequencies are in opposite phases to each other to provide null potential. On the other hand, even-order harmonics of a second harmonic or higher are combined for delivery. However, since higher harmonics of a fourth harmonic or higher have relatively low levels as compared with the second harmonic, the fundamental wave f0 and other harmonics are suppressed to supply second harmonic 2f0 on output line 4. Here, if the oscillator is designed to suppress second harmonic 2f0 as well, the oscillator can provide fourth harmonic 4f0 which has the next highest level.
Further, injection line 7 is injected with a synchronization signal at frequency f0/n, where n is an integer equal to or larger than one. This synchronization signal is injected into both oscillation systems in phase. This causes the oscillator to oscillate in synchronization with the synchronization signal, improving the frequency accuracy of the second-harmonic oscillator to as high as the frequency accuracy of the synchronization signal. For example, assuming n=1, fundamental wave f0 of each oscillation system is aligned in phase at time intervals of 1/f0, thus increasing the frequency stability of the oscillator following the frequency stability of a synchronization signal source. As such, the frequency stability can be improved for the second-harmonic oscillator by generating the synchronization signal from an oscillation source which exhibits a high frequency stability, such as a crystal oscillator.
Since slot line 2 is extended by a quarter wavelength relative to fundamental wave f0 from the upper and lower sections of microstrip line 1, the respective ends of slot line 2 are electrically open ends, viewed from the positions at which slot line 2 traverses microstrip line 1. Therefore, the oscillation component of fundamental wave f0 is efficiently transmitted to a positive feedback loop through slot line 2, thus increasing the Q-value of the oscillator circuit. The length λ/4, by which slot line 2 is extended, need not be strictly equal to λ/4 because this may be such a length that permits the ends of slot line 2 to be regarded as electrically open ends.
However, since the oscillation frequency cannot be made so high in the synchronization signal source, the foregoing injection locked second-harmonic oscillator is typically injected with a synchronization signal at frequency f0/n, where the value n is set to be two or more (n≧2). Consequently, the oscillator is aligned in phase at time intervals of 2/f0 or longer. In this event, the phase is left without synchronization for a longer period in which the phase can vary, possibly leading to a lower frequency stability, as compared with n=1, where the oscillator is aligned in phase at intervals of 1f0. Therefore, when n is set to two or more (n≧2), it is necessary to inject the synchronization signal at a higher level to increase the level of an f0 component contained in the synchronization signal as a harmonic, in order to increase the frequency stability. When the synchronization signal is injected at a higher level, phase noise is also improved in the fundamental frequency (fundamental wave f0) component of the oscillator. Also, since the oscillation frequency is more readily drawn into the synchronization signal, the oscillation frequency can be varied in response to the frequency of the synchronization signal source. In other words, the oscillation frequency can be drawn over a wider frequency range.
For the reason set forth above, a conventional injection locked high frequency oscillator employs a synchronization signal at f0/n, where n≧10, and a step recovery circuit or the like to increase harmonic components contained in the synchronization signal for purposes of increasing the frequency stability.
However, with the second-harmonic oscillator in the configuration described above, when n is set to two or more (n≧2) to increase the level of harmonic components contained in synchronization signal f0/n, fractional harmonic m×f0/n (m is an integer equal to or more than one) for fundamental wave f0 of the oscillator is generated by a similar push-push oscillation mechanism through the two oscillation systems of the second-harmonic oscillator. This causes a problem of increased spurious for a second harmonic component.